Key Concepts
Introduction
Structural changes: - Diaphragm elevated 4 cm (uterus pushes up) - Thoracic cage widens (subcostal angle increases from 68ยฐ to 103ยฐ) - These changes compensate โ overall tidal volume increases despite elevated diaphragm Lung volumes: - Tidal volume (TV): INCREASES 40โ50% (most important change) โ more air per breath - Residual volume (RV): decreases 20% โ air remaining after maximum exhalation - Functional residual capacity (FRC): decreases 20โ25% - Total lung capacity (TLC): slightly decreases (elevated diaphragm) - Vital capacity: unchanged or slightly increased - Respiratory rate: unchanged or minimally increased (12โ16/min) - Minute ventilation: INCREASES 40โ50% (TV ร RR) โ primary driver Gas exchange: - Progesterone stimulates respiratory center โ increases sensitivity to CO2 โ hyperventilation - Result: maternal PaCO2 decreases to 27โ32 mmHg (normal non-pregnant: 35โ45) - Compensatory metabolic acidosis: bicarbonate (HCO3โ) decreases to 18โ21 mEq/L - Result: mildly alkalotic ABG: pH 7.40โ7.47 - Maternal PaO2 increases slightly (to 104โ108 mmHg) - This creates a gradient favoring O2 transfer to fetus (fetal pO2 remains lower) Dyspnea of pregnancy: - Common complaint (~70% of pregnant women), especially 2nd trimester...
